This invention relates in general to electrophotographic imaging members and, more specifically, to layered photoreceptor structures with overcoatings containing stabilized hydrogen bonded materials and processes for making and using the photoreceptors.
Electrophotographic imaging members, i.e. photoreceptors, typically include a photoconductive layer formed on an electrically conductive substrate. The photoconductive layer is an insulator in the dark so that electric charges are retained on its surface. Upon exposure to light, the charge is dissipated.
Many advanced imaging systems are based on the use of small diameter photoreceptor drums. The use of small diameter drums places a premium on photoreceptor life. A major factor limiting photoreceptor life in copiers and printers, is wear. The use of small diameter drum photoreceptors exacerbates the wear problem because, for example, 3 to 10 revolutions are required to image a single letter size page. Multiple revolutions of a small diameter drum photoreceptor to reproduce a single letter size page can require up to 1 million cycles from the photoreceptor drum to obtain 100,000 prints, a desirable goal for commercial systems.
For low volume copiers and printers, bias charging rolls (BCR) are desirable because little or no ozone is produced during image cycling. However, the micro corona generated by the BCR during charging, damages the photoreceptor, resulting in rapid wear of the imaging surface, e.g., the exposed surface of the charge transport layer. For example wear rates can be as high as about 16 .mu. per 100,000 imaging cycles. Similar problems are encountered with bias transfer roll (BTR) systems. One approach to achieving longer photoreceptor drum life is to form a protective overcoat on the imaging surface, e.g. the charge transporting layer of a photoreceptor. This overcoat layer must satisfy many requirements, including transporting holes, resisting image deletion, resisting wear, avoidance of perturbation of underlying layers during coating. Although various hole transporting small molecules can be used in overcoating layers, one of the toughest overcoatings discovered comprises cross linked polyamide (e.g. Luckamide) containing N,N'-diphenyl-N,N'-bis(3-hydroxyphenyl)-[1,1'-biphenyl]-4,4'-diamine (DHTBD). This tough overcoat is described in U.S. Pat. No. 5,368,967, the entire disclosure thereof being incorporated herein by reference.
Since N,N'-diphenyl-N,N'-bis(3-hydroxyphenyl)-[1,1'-biphenyl]-4,4'-diamine is sensitive to the oxidative species produced by the various charging devices, a chemical stabilizer is desirable for longer imaging member cycling life. An improved overcoating has been achieved with cross linked polyamide (e.g., Luckamide) and N,N'-diphenyl-N,N'-bis(3-hydroxyphenyl)-[1,1'-biphenyl]-4,4'-diamine and bis-[2-methyl-4-(N-2-hydroxyethyl-N-ethyl-aminophenyl)]-phenylmethane (DHTPM) as an image deletion stabilizer material. Although excellent overcoatings have been achieved with bis-[2-methyl-4-(N-2-hydroxyethyl-N-ethyl-aminophenyl)]-phenylmethane as the stabilizer, bis-[2-methyl-4-(N-2-hydroxyethyl-N-ethyl-aminophenyl)]-phenylmethane is difficult to purify and handle. Moreover, it is expensive, a semi-solid at room temperature and oxidized relatively easily as evidenced by color change of the material during storage. However, since bis-[2-methyl-4-(N-2-hydroxyethyl-N-ethyl-aminophenyl)]-phenylmethane is soluble in alcohols, the solvents required for forming coatings containing polyamide (e.g. Luckamide), it can be solution coated with a polyamide.